Electronic phase shifting system



'- E INVENZF, l

ATTORNY www A A. E. CANFORA ELECTRONIC PHASE SHIFTING SYSTEM Aug. 14,1951 2 Sheets-Sheet 2 Filed April 29, 1947 INVENTOR.

ATTORNEY V Patented Aug. 14, 1951 ELECTRONIC PHASE SHIFTIN G SYSTEMArthur E. Canfora, Brooklyn,

N. Y., assignor to Radio Corporation of America, a corporation ofDelaware Application April 29, 1947, Serial No. 744,645

(Cl. Z50-27) 8 Claims. 1

My invention relates in gentral to circuits and apparatus for shiftingthe phase of an electrical wave formation and, more particularly, tosuch an apparatus that is entirely electronically controlled.

In certain systems for the transmission and reproduction ofintelligence, such, for instance, as in the art of facsimile and/orteletype transmission, it is necessary that synchronism be maintainedbetween a rotating device located at the transmitter and a rotatingdevice located at the receiver or reproducer. In the facsimile art, forexample, in one system, the recorded optical image to be transmitted isheld in a fixed position and a rotating optical system traverses therecorded image to scan it in a line by line manner. In anotherembodiment of this art, the recorded material to be transmitted may bewound about a drum in a fixed definite position and the drum may berotated so that line by line scansion is accomplished by an opticalsystem which is moved along parallel to the major axis of the drum. In-both of these systems mentioned for illustration, there is developed atthe end of each scansion line a signal which is indicative of theposition or phase of the optical system in the first illustrativeexample and of the drum in the second illustrative example. This signal,which is termed the line synchronizing signal, indicates theinstantaneous position of a. rotating element at the transmitter andwith which a rotating element at the receiver must be maintained insynchronisrn.

Reproduction of the signals developed by scansion at the transmitter isaccomplished line by line on a recording surface at the receiver. Inaccordance with one system, a conducting spiral or helical member orstrip is carried upon a recording drum member. A chemically treatedpaper, upon which the image record is to `be recreated, is inserted[between the spiral or helix member and a long bar-like electrode.Signals representative of the image to be produced in the record stripare impressed across the drum spiral or helix and the elongatedelectrode. As the drum is rotated, the current flow, representative ofthe signal, as it passes through the treated record strip or impregnatedpaper in owing between the bar and helix causes an electrolytic effectto take place within the record strip or paper to reproduce incrementalareas of the optica] image which are represented by the transmittedsignals. The record supporting drum of the receiver must be kept movingin synchronism with the rotating element at the transmitter so that aline by line scansion is accomplished which is similar to the analysiswhich occurs at the transmitter. Some representative examples offacsimile scanners and recorders may be found in U. S. Patent No. Re.20,152 to C. J. Young, granted October 27, 1936; No. 1,746,407, grantedFebruary 11, 1930, to Schroeder et al; No. 1,792,264 to Alexanderson,granted February 10, 1931; No. 2,215,806 to C. J. Young, grantedSeptember 24, 1940; No. 2,314,027, granted March 16, 1943, to C. J.Young; No. 2,354,571, granted July 25, 1944, to Blaim; No. 2,391,765,granted December 25, 1945, to Maurice Artzt; No. 2,394,649, grantedFebruary 12, 1946, to C. J. Young; and No. 2,413,400, granted December31, 1946, to C. J. Young.

Since the motor driving the rotating element for accomplishing scansionline by line at the transmitter and that driving the rotating element atthe receiver usually are not operated from the same power source, thereis generally a tendency on the part of one of the rotating elements todrift out of exact synchronous relationship with the other of therotating elements. Synchronous relationship between these two rotatingdevices usually is maintained by shifting the phase of the drivingsource at the receiver, when the standard sources of power, or thedrums, have drifted out of phase, by a predetermined number ofelectrical or mechanical degrees and this shift in phase is utilized tobring he drum at the receiver back into a synchronous relationship withrespect to the drum at the transmitter.

Among the presently known methods of and apparatus for accomplishing therequired shift in phase is one wherein the signal is compared in phasewith the phase of the power source at the receiver and a displacementbetween the correct phase and the actual phase causes the changing ofthe phase of the power source by mechanical means, in a phase shiftingnetwork. A second method of and apparatus for accomplishing the phaseshift utilizes the result of a phase comparison between 'the signal andthe source of power at the receiver to control, by mechanical means, therelative positioning between a plurality of inductors which are used inconjunction with a phase shift network to accomplish the desired shiftin phase.

A third method of and apparatus for accomplishing the shift in phasewhich is necessary to bring the receiving rotating element back intosynchronism with the rotating element at the transmitter is by utilizingthe result of a comparison between the received signals and the -powersource at the receiver to drive an auxiliary motor which, when it isactivated, physically turns the of an electrical wave formationy underthe control of correction pulses obtainedv by comparing the signals,such as synchronizingfsignals,.with the local power source, and whereinthe `phase shift is made in definite incremental steps of 90 each.

A still further object of this invention is to provide a method of andapparatus for shifting the phase of an electrical wave formation underthe control of correction pulses and wherein the phase shift: will`alwaysfbey inV a. desired direction, i. e., the phaseof thewaveformation will. always beeither advanced or retardedasfmay'be desired.

Other objects an'd advantages ofjmy. invention will be apparent: tothosey skilled in the art to which. this .invention belongs from areading of the hereinafter! appended specification'.

My invention'contemplates; in general, the` provision" of' an electricalwave. formation whose phase isto be'either. advanced or'retarded' byincremental fixed sums andthe provision of' a source of correctionpulsesunder the control of which the phase=shift willlbe:` accomplished. andwhich may be synchronizingfpulses"such as have been 'referred` to:hereinb'efore. If'thel wave formation whose phase maybe shiitedhas afrequencywhich is the samef asithat ofthe desired phase shifted". wave.thenT the available wave whose phaseV is" tov be: shifted mustiiirst bedoubled in frequency before itisfapplied tothe electronic phase shifterbecause'fy thei circuit inherently divides an applied frequency'by'two.This new frequency is' used-'to' control a.. multivibrator whose outputis alsol twice thei desired.. frequency.

There' is provided a? second: multivibrator which is" of themulti-stroke type' and in? which the: oute p-ut frequency therefrom islcnet-half.' that of` the input frequency. There are provided, iniconjunction with?v the second! multivibrator, two. control paths'v andthe conducting state of ther second multivibrator is controlled by theiirstl multivibrator in acclordanc'ewith which of the'two conductingpathsisoperated. In the exemplied embodiment of thisihventiomthese twoconducting paths comprise a;v pairA of. thermionic tubes and, onthe.control grid ofv each, there are impressed the outputpulsesifrom' one-of'the tubes of the first multivibrator: The conducting state of ea'chfVof these thermionic tubes is controlled by the conducting stateof. agaseous'discharge tube ofthe so-called th-yratron type', for instance,and the conducting state of the latter, inY turn, isrcontrolled by thecorrection pulses.

A circuit is provided` under the control of the second multivibratorwhich assures the control of the conducting state of thev gaseousdischarge tube by the correction pulses only during denite parts of` thecycle of operation of the secondrnultivibrator and, when the correctionpulses are allowed to control the gaseous discharge tube, a

phase shift ofthe output wave of' the second multivibrator of 90 isaccomplished. Thismay be -so` as to be in eitherdirection, i. e., thephase of 'the output wave of the secondfmulti-vibrator maylloe-advanced' by 90 or it may be retarded and each 4 change in phasethereafter will be in the same direction.

Hereinafter in this specification the grid controlled gaseous dischargetubes will be referred to as grid glow tubes.

My invention will best be understood by reference to the drawings inwhich,

Fig. 1' is a schematic v'diagram' of 1an apparatus for shifting theyphase' of a recurringwave by mechanically picking up a wave or waves ofdifferent phase relationships by rotating a condenser platewith respectto four xed plates;

Fig. 2 is a schematic showing of a so-called circuitrol element;

Fig. 3 showsranA embodiment of my invention; and,

Fig. v4, sectionsa. through d, are a set of explanatory curves. Y

Referring to Fig. 1, there is shown as an example of a setting for ,thepresent invention, a heretofore=utilized arrangement for' changingithephase of4 an elect-rical Wave in accordance with its relative'lphase.

Fig. 2 shows-a so-called"lcircuitrol arrangement for obtaining an outputwave whichV is shifted in phasel with respect to an input wave. Theapparatus used in conjunction w-ith the ar.- rangement. of- Fig. 2 wouldbe somewhatsimilar to the arrangement of the phasing` condenser shown inFig.V 1. The tube 273 and themotor- 24- of Fig. l areusedfwithfthearrangement of Fig..2;- The motor can be'usedto rotate theYinductance 29.

Referring to- Fig. 3, there is shownanv exemplii'led embodiment', of myinvention in the form of a circuit diagram. In thisdiagram, the'` apeparatushasbeen separated into= its component units bydashedanddottedflinesA in order toma-ke reierenceto.thevariouspartsthereof easier. In the arrangement of this-ligure;pulsesvhavinga definite frequency and- Which are to be changed in phaseunder certain circumstances, as-will be explained hereinafter, areimpressed' through condensers 3B and 3| ontothe-anodes of. two tubesA V1andI V2 forming a multivibrator. Each of the tubes haslthe anodeconnectedto a--source of positive potential throughappropriate-resistance elements and the g-rid of'eachA tube iscross.-connected tothe anode of. the opposing tube through an appropriatecapacitor. 'I-he'gr'idV of each tube is connected to the cathode44thereofthrough appropriate'resistance elements-and one side of thecathodeY of eachv tube is-l grounded. This is a conventional form ofmultivibrator and does not, of itself, form the essence of this invention and, accordingly, there is no necessity for adetailedexplanation-of its opera-tion'which is well know-n to thoseskilled in the artitow-hich this invention belongs.V In this particularinstance, the parametersof the circuit are chosen so that the outputfrequency ofthe utilized waves taken from the anode ofVi and that ofVz-.will-be equal to thatof the inputfrequency. Thismultivibratorunitwill be referred 'toi hereinafter as the unit H and it will beappreciated that two output signalsma-y be vderivedtherefrom which areof equal frequency and outfofphase with each other and: are, invgeneral, rectangular in wave shape.

There is provided a second multivibrator unit comprising thermionictubes V3 and V4 and the general construction of this multivibrator isthe same as that of the unit H, i. e., the multivibrator has the anodesof Vs'and V4 connected` to` al source y ofpositive potential throughresistance elements;

the grids of the tubes are cross-connected to the anode of the opposingtube through an appropriate capacitor and the grid of each tube isconnected to its cathode through an appropriate resistor and thecathodes are grounded. In this particular arrangement, the output wave,which will correspond in frequency to that of the Wave input to thefirst multivibrator whichcomprises the unit H but which will be phasedisplaced with respect thereto under certain conditions, may be takenfrom the second multivibrator from the anode of tube V2 and throughcoupling condenser 35 and between the terminals of the couplingcondenser and ground, as shown in the drawing. This multivibrator willbe referred to hereinafter as the unit J. The parameters of thismultivibrator are so chosen that it will act as a multi-strokemultivibrator, i. e., it will enter one portion of its cycle ofoperations by the impression of one control pulse thereon and willreturn to its initial state by the impression of a second pulse thereon.This then effectively comprises a frequency divider in which the outputwave is half the frequency of the control waves impressed thereon.

vThere are provided two vacuum tubes V and V5 both of which have theanodes thereof connected to a source of positive potential through anappropriate common resistance element 36. The anode of V5 is connectedto the anode of Vx through serially connected resistor 31 and condenser38. The anode of V5 is connected to the anode of V4 through seriallyconnected resistor 39 and condenser 40.

The grid G5 of tube V5 is connected to the anode of tube V2 throughcondenser 4|. The grid also has impressed thereon a negative potentialsupplied from a source indicated by the symbol -C and this is impressedonto the grid resistor R1. connected to the anode of tube V1 of the unitH through condenser 44 and this grid also is biased negatively by apotential source through a resistor R4.

There are provided two control tubes THi and THe which are grid glowtubes. Each of the tubes has the anode thereof connected to a source ofpositive potential through an appropriate resistance element,resistances R3 and Rs being used for this purpose. The anodes areconnected together through condenser C1. The anode of tube TH7 isconnected to the grid G5 of tube V5 through resistor R2 and theresistances R1, R2 comprise a Similarly, the grid G5 of vacuum tube Vsis brator comprising through resistor R5. The grid of V0 is connected tothe anode of V10 through condenser C5 and the grid is returned tocathode through resistor R9. The cathode is connected directly to asource of negative potential which is indicated by the symbol -C. Atapped point on resistor R5 is connected to the anode of tube V3 of theunit J through condenser C4.

There is further provided a thermionic tube V11 which acts as a controltube to influence the grid glow tubes of the unit K. The anode of tubeV11 is grounded through a condenser C2 and is also connected directly tothe cathode of a diode or other form of rectifier or detector D. Theanode of the diode or other rectifier D is grounded through seriallyconnected resistors 50 and 5|. Signal input to this diode or otherrectiiier is made between the common terminal of resistors 50 and 5| andthe ground point as indicated in the drawing. The input pulses which actas correction pulses are impressed onto the circuit in a positivepolarity as also indicated and it is these pulses which act toaccomplish the phase shift under certain conditions.

The cathode of tube V11 is grounded through resistor R10 and isconnected through condenser Cs to the common terminal of resistors and46 of the unit K.

The action of the circuit, which will be better understood withreference also to Fig. 4, is as follows:

Since the anodes of V5 and Vs are connected through appropriate couplingmembers to the anodes of tubes V3 and V4 respectively, pulses occurringin the anodes of V5 and Vs will change the operating state of themultivibrator comprising the unit J since this multivibrator is of themulti-stroke type, i. e., when it is in a definite portion of its cycle,its transition to the other portion of its cycle of operation may beaccomplished by pulses of correct polarity applied to the anode of oneof its tubes. In turn, the conducting state of tubes V5 and Vs iscontrolled by the conducting state of the tubes V1 and V2 of themultivithe unit H since the anode of V1 is connected to the grid of V5through capacitor 44 and the anode of V2 is connected to the grid of V5through capacitor 4l. The grids of V5 and V0 are biased by the negativesources of voltage divider circuit. Similarly, the anode of tube THS isconnected to the grid G5 of tube V6 through resistance member R5 and theresistances R4, R5 comprise a voltage divider similar to that ofresistances R1 and R2. The grid of tube THv is connected to the grid oftube THa through resistances 45 and 46, and the common terminal of theseresistances is joined to a source of negative potential identiiied bythe symbol -C through a resistance 47. The tubes V5 and Vs and the tubesTHv and THe, with their associated circuit elements, will be referred tohereinafter as the unit K and it is this unit which forms the controlfor accomplishing the effective phase shift of the Waves impressed ontothe multivibrator of the unit J.

A third multivibrator of the self-restoring type, or the one-stroketype, is provided and is identiiied as the unit L. This multivibratorcomprises thermionic tubes V9 and V10. The anode of V9 is connecteddirectly to the grid of V10 and the grid itself is connected to thecathode of V10 potential indicated by the symbol -C so that these tubesnormally would be maintained at just below their cut-off point in theabsence of any other potentials affecting their grids. It may beassumed, for purposes of illustration, that the grid glow tube TH7 is ina conducting state and that the grid glow tube THe is non-conducting orcut oif.

When this condition prevails, the grid of tube V5 is well below cut-oil`point due to the voltage divider action of resistors R1 and R2, thelatter being connected directly to the anode of grid glow tube TH'Iwhich is conducting. On the other hand, the grid of tube Vs will be at apotential p just below cut-off due to the voltage divider actube V6,

tion of resistors R4 and R5. Under these circumstances, positive pulsesfrom the multivibrator comprising the unit H may be impressed onto themultivibrator comprising the unit J only through since V5 is biasedconsiderably below the cut-oil point.

Referring to Figure 4a, there is shown the spacing of a series of pulsesimpressed onto tube Vs by the multivibrator comprising the unit H andonto which there is impressed the wave Whose phase is to be changedunder certain conditions. If the beginning thecycleisfassumedtobe: atime b.'

then.. the; multivibrator comprising. the:V unit: J.4

wouldY betriggeredbyv the; pulse occurring at the time tz and wouljdfcontinue in this state until it was triggeredby; the; pulseoccurring vatthe time ta, at which timeit Would-return to its originalstateandgthecycl'e would begin-again. Thepulse that occurred at the timet5. would trigger the the; time of pulsev P1 (which is supplied byVtheunit. H but which is ineffectiveV dueA tothe cut-G stagte o1.` V5),then the multivibrator comprising the unit J would be iiipped or sentinto thealter-l mating portion ofits, cycle or operation bythev pulseP1. For purposes of illustration, the;l time interval between the timet1.l and-the occurrence orthe pulse P1- hasgbeen identified as the timeinterval T1 and the time; interval between the occurrence-of pulse P1,which, really arrives from. the multivibrator unit H but is blocked bytube V5, andg the time tz, atwhich time' the transition inthe operatingstate of the multivibratorunit J actually takes place, is identied asthe interval T2.

The incomingpulses, which are usedas phasev shifting, pulses undercertain conditions, are impressed onto 'the` circuit including rectifierD, resistor` I` and condenser C21 Each ofthese pulses will store acharge incondenserC2 which ispositive; a-tl theterminal: thereof that isconnected tothe anode or tube-Vi1` due to therecticationthereofby-the-rectier-D. This-then supplies a positive potentialYtotheanodeof V11. For purposes ofillustration, let it be assumed thatthe gridzofY V11- isat a potential Wherebythe tube can conduct. Thismeans thatF the,` incoming'pulse, after rectication, will be conveyedviacoupling condenser-Cato the grids of grid: glow-tubes THv and THB. Ithas been assumed previously that THv was conducting at this-time. Theresult now will. bethat grid` glow tube THev will start to conduct. Theanode of- THv isat a lower potential than-.thatof- THS and, duetothe-storing of energyl in C1, the plate potential: of grid glow tube THvwill be` driven below its conducting point and THv will be extinguished;The conduction in THe will now cause-the grid potential of tube Vs to bedriven well below the cut-offpoint and, as avresult of the conduction ofTHa and the extinguishingy of tube THrr, the conduction sta-tes. of V5,and Vs are interchanged, This means then that the pulses suppliedfby theanode of tube Vzin the multivibrator of unitH willv controlthetransitionstates or the multivibrator comprising the. unit J and a reference toFig. 4sections band c thereof, will show that the transition point ofthe multivibrator of unit JJ is then shifted to coincide with the'linesP1, P3, and P4 of curve b of this iigure. It will bel seen, therefore,that the wave output ofv the multivibrator comprising unit J has beenadvanced one-quarter cycle or 90 with respect to its former transitionpoints which-would be P2, P5, etc. This will happen if the pulse, whichhas beenv impressed onto the rectifier D of unit M, is allowed to takeplace in the time T1.

On the other hand, if the impression of the pulse onto the multivibratorof unit J. in its as sumed state takes place during the interval Ta,

and thisis after the arrival of the pulse-P1 fromY theA multivibrator ofunit H, and which has been blocked due to the non-conducting state of-V5 atv this; time, the.phaseshiftingfpulse impressed. on therectier Dwill cause the reversal. of action of gridv glow tubesTHv and THS, thusblocking tube Vs and the neX-t pulsePz delivered from the unit H- willbeineffective and the pulse P3 will passl through tubev Veto. change thetransition state of` multivibrator'MVzwhich is the unit J. Reference tocurve cl 03E-Fig. Llwillshow, thatthe phase of. the output wave from`multivibrator MVz will have been retarded by 90.".

Where this. arrangement is used in a synchronizing system and where thepulses impressed on the rectifier D. may be the line synchronizingpulsesfroma facsimile transmitter, for instance, that might. arriveeither during they time T1 or T2 and thus with the circuit up to thispoint a randomshift of phase might take place, i. e., during one portionof the-shifting of phase the Wave output of MV2- might be advanced andbefore the twoy apparatuses were brought into synchronism, a retardingaction otphasemight be started. This, of course, isundesirableandthisisprevented by the 1 multivibrator. comprising unit L and which islof the self-restoring type. It` will benotedthat the anode of tube. V9of this unit is connected directly to the grid of. tube V11 of unit M.Additionally, the grid of tube V9 is connected through appropriatediferentiating condenser C4 to the anode oi tube V3 of multivibrator MVzwhich is the unit J. OnceY per cycleA of the output. wave of themultivibrator MV2 of Vunit J, the negative pulses from the tube V3 willbe diierentated by condenser Q1., These pulsesl are impressed ontor thegrid circuit of; V11V and will cut on V9 whichis. normally conductingandbyl proper. choice of theA value.

of Re. andY C5 this conditiony is maintained for a time which is,approximately equal to the time T1 .Y DuringV the time, interval T1,tube V9 being non-conducting, there will be no potential drop acrossresistor Raand the grid of tube V11, which is connected to thisresistor, will have apotential which is approximately groundpotential,yand,y accordingly, when a pulse is impressed onto4 the detector D,during4 this interval, the plate voltage of' this tube will rise to thedegree where a potential pulse occurring across cathode resistor R10 maybe fed through condenserl C3l to affect the conducting states of gridglow tubes THe and THB, Alternatively, during the time T2, the tube Vowill be conducting and the potential drop acrossv resistor Ra, due tothe anode current of this tube, will be great enough to drive the gridofA tube V11 sufficiently negative so as to be fairly well below thecut-off point. Accordingly, it is obvious that the pulse impressedy onrectiiier D can be eective only during the time of the cycle representedby T1. A phase shifting pulse received at a time other than during theinterval T1 wiljl be storedA by condenser C2 and will cause apulsethrough V11 only during such time as tube V9 is non-conducting and thisis during the inter- Val T1.

It is possible that some applications of phase shifting might requirethat the Wave be retarded instead of advanced in phase. This can beaccomplished by assuring'that the switching action of the unit L beperformed during the time T2 instead of during the time T1 as isillustrated in the drawings. This could be accomplished by controllingthe trigger circuit by the anodes of scatto the multivibrator MV1comprising unit I-I and switching from one anode to the other after eachphase shift by means of the grid glowtubes TH': and THS and anladditional set of triodes controlled in the same manner that V5 and V6are controlled.

It is also possible to advance or retard the wave form provided twoseparate trains of correction pulses are available or with a singletrain of pulses having opposite polarities for an advancing or retardingof thewave whose phase is to be shifted. Y

It will befappreciated that the alternative could be arranged veryeasily and that tube V11 could be controlled so as to make the pulseseffective only during the time T2 and hence the phase of themultivibrator MVz would be changed in incremental steps of 90 which were96 behind the phase of the input wave to the multivibrator comprisingunit H. This will occur until the phase of the control pulses impressedonto the rectifier D bears a selected relationship to the phase of theinput pulses to multivibrator MV1. The output signals taken frommultivibrator MVz can, after power amplification, be used to drive adriving motor associated with a facsimile apparatus or a teletype, forinstance.

Having now described the invention, what is claimed and desired to besecured by Letters Patent is the following:

l. A phase shifting apparatus comprising a multivibrator, a plurality ofswitch means connected to said multivibrator for impression of pulsesthereon for controlling the transition times at which the multivibratoris changed from one conducting state into another, means to impress thewave whose phase is to be shifted onto said switch means, a source ofcorrection pulses, and means operable under the iniiuence of saidcorrection pulses to select the switch means through which the wavewhose phase is to be shifted is impressed onto said multivibratorcircuit.

2. A phase shifting apparatus comprising a multivibrator, a plurality ofswitch means connected to said multivibrator to impress pulses thereonfor controlling the transition times at which the multivibrator ischanged from one conducting state into another, means to impress thewave whose phase is to be shifted onto said switch means, a source ofcorrection pulses, and means operable under the iniiuence of saidcorrection pulses at selected intervals to select the switch meansthrough which the wave whose phase is to be shifted is impressed ontosaid multivibrator circuit.

3. A phase shifting apparatus comprising a source of wave energy, amultivibrator having two predetermined states of conduction, switchmeans interposed between the multivibrator and the source of waveenergy, means for impressing onto the switch means the wave energy whosephase is to be shifted, thermionic means connected to said switch meansto control the operating level thereof and to select the switch meansthrough which the source of Wave energy may control the transition timesin the conducting states of the multivibrator, a source of correctionpulses, and means for controlling the conduction state of saidthermionic means under the influence of the correction pulses.

4. A phase shifter comprising a source of wave energy, a multivibratorhaving two operating conduction states, a vacuum tube connected betweenthe source of wave energy and each tube included in the multivibratorcircuit for con- 1o trolling the transition times and changing theconduction states ofthe multivibrator, a thermionic tube connected so asto control the operating level of each of said vacuum tubes, a sourceVof correction pulses, and means for impressing said correction pulsesonto said latter thermionic tubes for controlling the conduction stateof said thermionic tubes and thereby influencing the transition times inthe conduction states of the multivibrator.

5. Apparatus in accordance with claim 4 wherein said thermionic tubesconnected so as to control the operating level of each of the vacuumtubes comprises a plurality of grid glow tubes so interconnected thatthe application of a correction pulse onto said grid glow tubes will reone of them and cut off the other.

6. Apparatus in accordance with claim 4 wherein the means for impressingthe correction pulses onto the thermionic tubes comprises a circuitincluding a rectifier, electrical energy storage means connected so asto have energy delivered by said rectier stored therein, means forimpressing the correction pulses onto said rectifier, and means couplingsaid electrical energy storage means to the thermionic tubes to deliverpulses from said electrical energy storage means and to control theconduction states of said thermionic tubes.

'7. Apparatus in accordance with claim 6 wherein the means coupling theelectrical energy storage means to the thermionic tubes comprises atriode normally biased to cut-olf, and means under the iniluence of theoutput of said multivibrator for rendering the triode conductive atselected intervals.

8. A phase shifting apparatus comprising a source of wave energy, a rstmultivibrator having two thermionic tubes cross-connected so as to alloweach to iniluence the conducting state of the other and having anoperating frequency which is twice that of the output frequency of theapparatus, a second multivibrator circuit having two thermionic tubescross-connected so that each influences the conducting state of theother, a rst thermionic tube having anode, cathode and controlelectrodes, a second thermionic tube having anode, cathode and controlelectrodes, means for impressing pulses delivered by one of the tubes ofthe first multivibrator onto the control grid of the first thermionictube, means for impressing the pulses delivered by the other tube of therst multivibrator onto the control grid of the second thermionic tube, arst grid glow tube, a second grid glow tube, means connecting the anodeof the first grid glow tube to the grid of the first thermionic tube soas to control the operating level of the rst thermionic tube, meansconnecting the anode of the second grid glow tube to the control grid ofthe second thermionic tube so as to control the operating level of thesecond thermionic tube, means coupling the anodes of said grid glowtubes whereby a pulse occurring in the anode of one of said grid glowtubes is impressed onto the anode of the other of said grid glow tubes,a source of correction pulses, a detector, electrical energy Ystoragemeans connected to said detector so as to have rectied energy storedtherein, means for impressing the correction pulses onto said detectorso as to rectify the pulses and store energy in said electrical energystorage means, a third thermionic tube having anode, cathode and gridelectrodes, means connecting the anode of said third thermionic tube tosaid detector and to said electrical energy storage means whereby theanode o f said tube circuitof saidthird'v thermionic tbe'to the control'gridsV of both ofv saidfgrid g'lo'wtules,V a,4

trigger circuit having two tubes cross-connected so"asto have twoconducti'states',ineens foi" controlling the conduction State of 44saidtrigger circuit under the influence `of signals from said secondmultivibrator, and' means' to control the conducting state of the thirdthermonic tube un# der `the influence of "said trigger' 'circuit'.`

- "'ARTHUR'ECANFORA.

